What kind of mods are you talking about? There are vehicle mods, weapon mods, script mods in .asi format for ScripthookV, script mods in .dll format for ScriptVDotNet, script mods in .dll format for the RAGEpluginHook and more. They all need to be installed in a different way.

It's always a good idea to have a closer look at the zip files (or rar) you have downloaded. Usually they contain a README which explains how to install that mod.

Please use a more descriptive thread title in the future. Just "help" looks kind of lazy and is no great motivation to even open such a thread.